As the data rate in a digitally modulated wireless communication system is increased, a corresponding increase in the output power of the signals radiated by a tower-mounted antenna is typically required to effectively communicate with subscribers within a given service area. Thus, migrating an existing system to a higher data rate often requires more output power from the amplifiers used in the system and/or a reduction or elimination of losses associated with components in the system. However, it has been found that certain known modulation schemes may be better suited for migrating to higher data rates than others as the ability to increase output power differs for various modulation schemes.
For example, in systems using code-division multiple access (CDMA or WCDMA) modulation, a single multi-carrier amplifier may be used for several different carriers. In order to provide the additional radiated output power associated with higher data rates, a single, large multi-carrier amplifier is used in the system. Thus, a multi-carrier amplifier allows the task of amplifying the broad frequency spectrum associated with several carriers using a single, high power linear amplifier. As a result, multi-carrier amplifiers configured for use in CDMA systems may be capable of providing the additional radiated output power associated with higher data rates.
In other environments, comparable results may be obtained through a reduction of losses. For example, in systems using time-division multiple access (TDMA) modulation, a tunable cavity combiner, which typically has a relatively low insertion loss, may often be used to reduce losses, thereby requiring less gain from any amplifiers used therewith, and possibly providing the additional radiated output power associated with a higher data rates and multiple carriers.
Other modulation schemes, however, are not as well suited to increasing output power and carriers merely through the use of additional amplifiers dedicated to individual carriers or low insertion loss combiners. For example, unlike CDMA and TDMA systems, Global System for Mobile (GSM) communications systems use frequency hopping techniques to minimize interference between adjacent channels. Thus, unlike in a CDMA or TDMA system, the active carriers in GSM system may dynamically change from time to time, a process commonly referred to as frequency hopping. Therefore, amplifiers and combiners used with a GSM system may require greater bandwidth than those used in a CDMA or TDMA system to allow for frequency hopping.
Due to the requirement of greater bandwidth, multi-carrier power amplifiers and tuned cavity combiners are not as well suited for use in GSM systems. In particular, constructing a multi-carrier amplifier wherein each amplifier is capable of uniformly amplifying the bandwidth associated with frequency hopping in a GSM system can be expensive. Similarly, constructing a wide bandwidth tuned cavity combiner with low insertion loss across the band is difficult since the cavity is often optimized for a particular frequency to achieve low insertion loss. As a result, GSM systems often use hybrid combining due to bandwidth considerations associated with frequency hopping. However, a power loss of 3 dB is typically associated with hybrid combining, requiring even more gain and output power from amplifiers used therewith.
Recently, a new modulation technique was released for GSM communications referred to as Enhanced Data rates for Global Evolution, or EDGE. EDGE allows network operators to use existing GSM infrastructure to provide data, multimedia, and application services at rates of up to 384 kilobits per second (kbps), more than three times the speed of GSM. A difficulty encountered using existing GSM infrastructure to provide EDGE services is that EDGE modulation requires an additional 3-4 decibels (dB) more radiated power output than typical GSM systems.
In order to provide the additional gain necessary in providing higher data rates services, such as EDGE, some network operators have recognized the losses associated with hybrid combining and have resorted to using GSM multi-carrier power amplifiers. However, multi-carrier power amplifiers for such systems may be prohibitively expensive for some service providers in adapting their systems to high data rate modulation schemes, such as EDGE.
Thus, there is a need for an economical alternative that allows network operators to provide higher data rate services, such as EDGE, by affording additional gain and power through avoiding the losses associated with combiners typically used in such systems, and without resorting to using expensive multi-carrier amplifiers.